1. Field of the Invention
The present invention relates to audio source coding systems but the same methods could also be applied in many other technical fields.
2. Description of Related Art
Recent development in audio coding has made available the ability to recreate a multi-channel representation of an audio signal based on a stereo (or mono) signal and corresponding control data. These methods differ substantially from older matrix based solution such as Dolby Prologic, since additional control data is transmitted to control the re-creation, also referred to as up-mix, of the surround channels based on the transmitted mono or stereo channels.
Hence, the parametric multi-channel audio decoders reconstruct N channels based on M transmitted channels, where N>M, and the additional control data. The additional control data represents a significant lower data rate than transmitting all N channels, making the coding very efficient while at the same time ensuring compatibility with both M channel devices and N channel devices.
These parametric surround coding methods usually comprise a parameterisation of the surround signal based on IID (Inter channel Intensity Difference) and ICC (Inter Channel Coherence). These parameters, commonly referred to as “spatial parameters” describe power ratios and correlation between channel pairs in the up-mix process. Further parameters also used in prior art comprise prediction parameters used to predict intermediate or output channels during the up-mix procedure.
The spatial parameters can be extracted in several ways. One beneficial method as known in prior art, is to device several encoding modules that take two original input signals and produces an output signal, being a downmix of the two input signals, and the corresponding spatial parameters required to recreate an accurate replica of the two original signals based on the mono downmix and the spatial parameters. Another commonly used encoding module requires three input signals, and produces two output signals and the corresponding spatial parameters.
These modules can be connected in what is commonly referred to as a tree structure, where the output of one module is input to another module.
The above mentioned parameters need to be coded efficiently, in order to get the lowest bitrate over-head. At the same time, an optimum choice between frequency resolution and time resolution needs to be made. The present invention teaches several ways to optimize the trade-off between time and frequency resolution of the spatial parameters, as well as introducing novel methods for signalling, and controlling interpolation of the parameters.